Pixel circuit, active matrix apparatus and display apparatus

ABSTRACT

A pixel circuit having a function of compensating for characteristic variation of an electro-optical element and threshold voltage variation of a transistor is formed from a reduced number of component elements. The pixel circuit includes an electro-optical element, a holding capacitor, and five N-channel thin film transistors including a sampling transistor, a drive transistor, a switching transistor, and first and second detection transistors. The sampling transistor samples and supplies an input signal from a signal line so as to be held into the holding capacitor. The driving transistor drives the electro-optical element with current in response to the held signal potential. The first and second detection transistors detect a threshold voltage of the drive transistor and supply the detected voltage into the holding capacitor in order to cancel an influence of the threshold voltage in advance.

BACKGROUND OF THE INVENTION

This invention relates to a pixel circuit wherein a load elementdisposed for each pixel is driven by current and also to a matrixapparatus wherein a plurality of pixel circuits are disposed in a matrixand particularly to an active matrix apparatus wherein an amount ofcurrent to be supplied to a load element is controlled by an insulatinggate type field effective transistor provided in each pixel circuit. Thepresent invention further relates to a display apparatus of the activematrix type which includes an electro-optical element whose luminance iscontrolled by a value of current such as an organic EL element as a loadelement.

In an image display apparatus such as, for example, a liquid crystaldisplay apparatus, a large number of liquid crystal elements arejuxtaposed in a matrix and the transmission intensity or reflectionintensity of incoming light is controlled for each pixel in response toimage information to be displayed thereby to display an image. Althoughthis similarly applies to an organic EL display apparatus which uses anorganic EL element for a pixel or a like apparatus, different from aliquid crystal element, an organic EL element is a self light emittingelement. Therefore, the organic EL display apparatus is advantageous inthat the image displayed thereon is higher in visibility than that onthe liquid crystal display apparatus, that no backlight is required andthat the responding speed is high. Further, the organic EL displayapparatus is much different from the liquid crystal display apparatus inthat the luminance level (gradation) of each light emitting element isof the current controlled type wherein it can be controlled by the valueof current flowing therethrough.

For the organic EL display apparatus, two different driving methods areavailable including a simple matrix type driving method and an activematrix type driving method similarly to the liquid crystal displayapparatus. The former has a problem that implementation of a displayapparatus of a large size and a high definition is difficult although itis simple in structure. Therefore, development of organic EL displayapparatus which uses the active matrix type driving method is proceedingenergetically. According to the active matrix type driving method,current to flow to a light emitting element in the inside of each pixelcircuit is controlled by an active element (usually a thin filmtransistor: TFT) provided in the pixel circuit.

Organic EL display apparatus of the type described are disclosed, forexample, in Japanese Patent Laid-Open Nos. 2003-255856 and 2003-271095.

FIG. 10 shows a configuration of an exemplary organic EL displayapparatus. Referring to FIG. 10, the display apparatus 100 shownincludes a pixel array section 102 in which pixel circuits (PXLC) 101are arranged in an m×n matrix, a horizontal selector (HSEL) 103, a writescanner (WSCN) 104, and a drive scanner (DSCN) 105. The displayapparatus 100 further includes signal lines DTL101 to DTL10 n for beingselected by the horizontal selector 103 such that a signal based onluminance information is supplied thereto, scanning lines WSL101 toWSL10 m for being selectively driven by the write scanner 104, andscanning lines DSL101 to DSL10 m for being selectively driving by thedrive scanner 105.

FIG. 11 shows an example of a configuration of a pixel circuit shown inFIG. 10. Referring to FIG. 11, the pixel circuit 101 shown is basicallyformed using a thin film field effect transistor (hereinafter referredto as TFT) of the p-channel type. In particular, the pixel circuit 101includes a drive TFT 111, a switching TFT 112, a sampling TFT 115, anorganic EL element 117, and a holding capacitor C111. The pixel circuit101 formed from the elements mentioned is disclosed at an intersectingpoint of a signal line DTL101 with a scanning line WSL101 and a signalline DTL101. The signal line DTL101 is connected to the drain of thesampling TFT 115 while the scanning line WSL101 is connected to the gateof the sampling TFT 115, and the other scanning line DSL101 is connectedto the gate of the switching TFT 112.

The drive TFT 111, switching TFT 112 and organic EL element 117 areconnected in series between a power supply potential Vcc and a groundpotential GND. In particular, the source of the drive TFT 111 isconnected to the power supply potential Vcc, and the cathode of theorganic EL element 117 (light emitting element) is connected to theground potential GND. Since the organic EL element 117 generally has arectifying action, it is represented by a mark of a diode. Meanwhile,the sampling TFT 115 and the holding capacitor C111 are connected to thegate of the drive TFT 111. The gate-source voltage of the drive TFT 111is represented by Vgs.

In operation of the pixel circuit 101, the scanning line WSL101 is firstplaced into a selection condition (here, the low level) and a signal isapplied to the signal line DTL101. Thereupon, the sampling TFT 115 isrendered conducting so that the signal is written into the holdingcapacitor C111. The signal potential written in the holding capacitorC111 acts as the gate potential to the drive TFT 111. Then, the scanningline WSL101 is placed into a non-selection state (here, the high level).Consequently, the signal line DTL101 and the drive TFT 111 areelectrically disconnected from each other. However, the gate potentialVgs of the drive TFT 111 is held stably by the holding capacitor C111.Thereafter, the other scanning line DSL101 is placed into a selectionstate (here, the low level). Consequently, the switching TFT 112 isrendered conducting, and driving current flows from the power supplypotential Vcc toward the ground potential GND through the TFTs 111 and112 and light emitting element 117. Then, when the scanning line DSL101is placed into a non-selection state, the switching TFT 112 is turnedoff, and the driving current does not flow any more. The switching TFT112 is inserted in order to control the time of light emission of thelight emitting element 117.

The current flowing through the TFT 111 and the light emitting element117 has a value corresponding to the gate-source voltage Vgs of thedrive TFT 111, and the light emitting element 117 continues to emitlight with a luminance corresponding to the current value. Suchoperation of selecting the scanning line WSL101 to transmit a signalapplied to the signal line DTL101 to the inside of the pixel circuit 101is hereinafter referred to as “writing”. If writing of a signal isperformed once as described above, then the light emitting element 117continues to emit light with a fixed luminance for a period of timeuntil writing into the organic EL element 117 is performed subsequently.

As described above, the value of current to flow to the light emittingelement 117 is controlled by adjusting the voltage to be applied to thegate of the TFT 111 serving as a drive transistor in response to aninput signal. At this time, since the source of the p-channel drivetransistor 111 is connected to the power supply potential Vcc, the TFT111 normally operates in a saturation region. Consequently, the drivetransistor 111 serves as a current source having a current value givenby the following expression (1):Ids=(½).μ.(W/L).Cox.(Vgs−Vth)2  (1)where Ids is the current flowing between the drain-source of thetransistor which operates in a saturation region, μ the mobility, W thechannel width, L the channel length, Cox the gate capacitance, and Vththe threshold value of the transistor. As apparent from the expression(1), in a saturation region of the transistor, the drain current Ids ofthe transistor is controlled by the gate-source voltage Vgs. Since thegate-source voltage Vgs of the drive transistor 111 shown in FIG. 9 isheld fixed, the drive transistor 111 operates as a constant currentsource and can cause the light emitting element 117 to emit light with afixed luminance.

FIG. 12 is a graph illustrating aged deterioration of thecurrent-voltage (I-V) characteristic of an organic EL element. In thegraph, a curve indicated by a solid line represents the characteristicin an initial state, and another curve which is indicated by a brokenline represents the characteristic after aged deterioration. Usually,the I-V characteristic of an organic EL element deteriorates with timeas seen from the graph. However, in the pixel circuit shown in FIG. 11,since the drive transistor is driven by constant current, the draincurrent Ids continues to flow through the organic EL element, and evenif the I-V characteristic of the organic EL element deteriorates, theluminance of emitted light of the organic EL element does notdeteriorate with time.

SUMMARY OF THE INVENTION

While the pixel circuit shown in FIG. 11 is formed using a p-channelTFT, if it can be formed otherwise using an n-channel TFT, then aconventional amorphous silicon (a-Si) process can be applied to TFTproduction. This makes it possible to reduce the cost of a TFTsubstrate, and it is expected to develop a pixel circuit formed using ann-channel TFT.

FIG. 13 is a circuit diagram showing a configuration wherein thep-channel TFTs of the pixel circuit shown in FIG. 11 are replaced byn-channel TFTs. Referring to FIG. 13, the pixel circuit 101 shownincludes n-channel TFTs 111, 112 and 115, a holding capacitor C111 andan organic EL element 117 which is a light emitting element. The TFT 111is a drive transistor, and the TFT 112 is a switching transistor whilethe TFT 115 is a sampling transistor. Further, in FIG. 13, referencecharacter DTL101 denotes a signal line, and reference characters DSL101and WSL101 denote each a scanning line. Further, in the pixel circuit101, the drain of the TFT 111 as a drive transistor is connected to apower supply potential Vcc and the source of the TFT 111 is connected tothe anode of the organic EL element 117 thereby to form a sourcefollower circuit.

FIG. 14 is a timing chart illustrating operation of the pixel circuitshown in FIG. 13. Referring to FIG. 14, if a selection pulse is appliedto the scanning line WSL101, then the sampling TFT 115 is renderedconducting and samples and writes a signal from the signal line DTL101into the holding capacitor C111. Consequently, the gate potential of thedrive TFT 111 is held at the sampled signal potential. This samplingoperation is performed line sequentially. In particular, after aselection pulse is applied to the scanning line WSL101 of the first row,another selection pulse is applied to the scanning line WSL102 of thesecond row, and thereafter, pixels for one row are selected for each onehorizontal period (1H). Since also the scanning line DSL101 is selectedsimultaneously with the selection of the scanning line WSL101, theswitching TFT 112 is turned on. Consequently, driving current flows tothe light emitting element 117 through the drive TFT 111 and theswitching TFT 112 so that light is emitted from the light emittingelement 117. Intermediately within one field period (if), the scanningline DSL101 is placed into a non-selection state, and the switching TFT112 is turned off. Consequently, the emission of light is stopped. Thescanning line DSL101 controls the period of time (duty) of lightemission which occupies the one field period.

FIG. 15A is a graph illustrating a working point of the drive transistor111 and the EL element 117 in the initial state. Referring to FIG. 13 a,the axis of abscissa indicates the drain-source voltage Vds of the drivetransistor 111, and the axis of ordinate indicates the drain currentIds. As seen in FIG. 13 a, the source potential depends upon the workingpoint of the drive transistor 111 and the EL element 117, and thevoltage of the source potential has a value which is different dependingupon the gate voltage. Since the drive transistor 111 is driven in thesaturation region, the drain current Ids of the current value defined bythe expression (1) given hereinabove with respect to the gate-sourcevoltage Vgs corresponding to the source voltage of the working point issupplied.

However, the I-V characteristic of the EL element deteriorates with timeas described hereinabove. As seen in FIG. 15B, the aged deteriorationchanges the working point, and even if an equal gate voltage is applied,the source voltage of the transistor changes. Consequently, thegate-source voltage Vgs of the drive transistor 111 changes, and thevalue of flowing current varies. Simultaneously, also the value ofcurrent flowing though the EL element 117 varies. In this manner, thepixel circuit of a source follower configuration shown in FIG. 13 has asubject to be solved that, if the I-V characteristic of the organic ELelement changes, then the luminance of light emission of the organic ELelement varies with time.

It is to be noted that also it is a possible idea to dispose the driveTFT 111 and the EL element 117 reversely in order to eliminate thesubject described above. In particular, according to the possiblecircuit configuration just mentioned, the source of the drive transistor111 is connected to the ground potential GND and the drain of the drivetransistor 111 is connected to the cathode of the EL element 117 whilethe anode of the EL element 117 is connected to the power supplypotential Vcc. In the circuit configuration described, the potential ofthe source of the drive transistor 111 is fixed and the drive transistor111 operates as a constant current source similarly as in the pixelcircuit of the p-channel TFT configuration described hereinabove withreference to FIG. 11. Consequently, also a luminance variation bydeterioration of the I-V characteristic of the EL element can beprevented. However, according to the circuit configuration, it isnecessary to connect the drive transistor to the cathode side of the ELelement. Such cathode connection requires development of a novel anodeelectrode and cathode electrode, and it is considered that this is verydifficult with the technique at present. From the foregoing situation,the conventional technique fails to place an organic EL displayapparatus which uses an n-channel transistor and does not exhibit aluminance variation into practical use.

In an organic EL display apparatus of the active matrix type, also thethreshold voltage of n-channel TFTs which form the pixel circuit varieswith time in addition to the characteristic variation of the EL element.As apparent from the expression (1) given hereinabove, if the thresholdvoltage Vth of the drive transistor varies, then the drain current Idschanges. Consequently, there is a subject to be solved that theluminance of emitted light varies by variation of the threshold voltageVth.

Therefor, it is desirable to provide a pixel circuit by which theluminance of light to be emitted can be kept fixed even if the I-Vcharacteristic of a load element of the current driven type (anelectro-optical element such as, for example, an organic EL element)such as a light emitting element varies with time.

It is also desirable to provide a pixel circuit wherein a load elementcan be driven stably even if the threshold voltage of a transistor whichforms the pixel circuit varies with time.

It is further desirable to provide a pixel circuit having a function ofcompensating for a characteristic variation of a load element andanother function of compensating for a variation of the thresholdvoltage of a transistor wherein the number of circuit componentsnecessary to provide the compensation functions is reduced to theutmost.

In order to attain the desire described above, according to anembodiment of the present invention, there is provided a pixel circuitdisposed at a point at which a scanning line and a signal line intersectwith each other, including an electro-optical element, a holdingcapacitor, and five N-channel thin film transistors including a samplingtransistor, a drive transistor, a switching transistor, a firstdetection transistor and a second detection transistor, wherein, theholding capacitor being connected between the source and the gate of thedrive transistor, the electro-optical element being connected betweenthe source of the drive transistor and a predetermined cathodepotential, the first detection transistor being connected between thesource of the drive transistor and a first ground potential, the seconddetection transistor being connected between the gate of the drivetransistor and a second ground potential, the sampling transistor beingconnected between the gate of the drive transistor and the signal line,the switching transistor being connected between the drain of the drivetransistor and a predetermined power supply potential, the samplingtransistor operating, when selected by the scanning line, to sample aninput signal from the signal line and supply the sampled input signal soas to be held into the holding capacitor, the drive transistor drivingthe electro-optical element with current in response to the signalpotential held by the holding capacitor, the switching transistor beingrendered conducting, when selected by the scanning line, to supplycurrent from the power supply potential to the drive transistor, and thefirst and second detection transistors operating, when selected by thescanning line, to detect a threshold voltage of the drive transistorprior to the driving of the electro-optical element with the current andsupply the detected potential so as to be held into the holdingcapacitor in order to cancel an influence of the threshold voltage inadvance.

According to another embodiment of the present invention, there isprovided an active matrix apparatus including a plurality of scanninglines extending along rows, a plurality of signal lines extending alongcolumns, and a plurality of pixels disposed in a matrix at points atwhich the scanning lines and the signal lines intersect with each other,wherein, each of the pixels including a load element, a holdingcapacitor, and five N-channel thin film transistors including a samplingtransistor, a drive transistor, a switching transistor, a firstdetection transistor and a second detection transistor, the holdingcapacitor being connected between the source and the gate of the drivetransistor, the load element being connected between the source of thedrive transistor and a predetermined cathode potential, the firstdetection transistor being connected between the source of the drivetransistor and a first ground potential, the second detection transistorbeing connected between the gate of the drive transistor and a secondground potential, the sampling transistor being connected between thegate of the drive transistor and the signal line, the switchingtransistor being connected between the drain of the drive transistor anda predetermined power supply potential, the sampling transistoroperating, when selected by the scanning line, to sample an input signalfrom the signal line and supply the sampled input signal so as to beheld into the holding capacitor, the drive transistor driving the loadelement with current in response to the signal potential held by theholding capacitor, the switching transistor being rendered conducting,when selected by the scanning line, to supply current from the powersupply potential to the drive transistor, and the first and seconddetection transistors operating, when selected by the scanning line, todetect a threshold voltage of the drive transistor prior to the drivingof the load element with the current and supply the detected potentialso as to be held into the holding capacitor in order to cancel aninfluence of the threshold voltage in advance.

According to a further embodiment of the present invention, there isprovided a display apparatus including a plurality of scanning linesextending along rows, a plurality of signal lines extending alongcolumns, and a plurality of pixels disposed in a matrix at points atwhich the scanning lines and the signal lines intersect with each other,wherein, each of the pixels including an organic electroluminescenceelement, a holding capacitor, and five N-channel thin film transistorsincluding a sampling transistor, a drive transistor, a switchingtransistor, a first detection transistor and a second detectiontransistor, the holding capacitor being connected between the source andthe gate of the drive transistor, the organic electroluminescenceelement being connected between the source of the drive transistor and apredetermined cathode potential, the first detection transistor beingconnected between the source of the drive transistor and a first groundpotential, the second detection transistor being connected between thegate of the drive transistor and a second ground potential, the samplingtransistor being connected between the gate of the drive transistor andthe signal line, the switching transistor being connected between thedrain of the drive transistor and a predetermined power supplypotential, the sampling transistor operating, when selected by thescanning line, to sample an input signal from the signal line and supplythe sampled input signal so as to be held into the holding capacitor,the drive transistor driving the organic electroluminescence elementwith current in response to the signal potential held by the holdingcapacitor, the switching transistor being rendered conducting, whenselected by the scanning line, to supply current from the power supplypotential to the drive transistor, and the first and second detectiontransistors operating, when selected by the scanning line, to detect athreshold voltage of the drive transistor prior to the driving of theorganic electroluminescence element with the current and supply thedetected potential so as to be held into the holding capacitor in orderto cancel an influence of the threshold voltage in advance.

According to a still further embodiment of the present invention, thereis provided a pixel circuit disposed at a point at which a scanning lineand a signal line intersect with each other, including anelectro-optical element, a holding capacitor, and five N-channel thinfilm transistors including a sampling transistor, a drive transistor, aswitching transistor, a first detection transistor and a seconddetection transistor, wherein, the drive transistor being connected atthe gate thereof to an input node, at the source thereof to an outputnode and at the drain thereof to a predetermined power supply voltage,the electro-optical element being connected between the output node anda predetermined cathode potential, the holding capacitor being connectedbetween the output node and the input node, the sampling transistorbeing connected to the input node and the signal line, the firstdetection transistor being connected between the output node and a firstground potential, the second detection transistor being connectedbetween the input node and a second ground potential, the switchingtransistor being interposed between the input node and the gate of thedrive transistor, the sampling transistor operating, when selected bythe scanning line, to sample an input signal from the signal line andsupply the sampled input signal so as to be held into the holdingcapacitor, the switching transistor being rendered conducting, whenselected by the scanning line, to connect the holding capacitor to thegate of the drive transistor, the drive transistor driving theelectro-optical element with current in response to the signal potentialheld by the holding capacitor, and the first and second detectiontransistors operating, when selected by the scanning line, to detect athreshold voltage of the drive transistor prior to the driving of theelectro-optical element with the current and supply the detectedpotential so as to be held into the holding capacitor in order to cancelan influence of the threshold voltage in advance.

According to a yet further embodiment of the present invention, there isprovided an active matrix apparatus including a plurality of scanninglines extending along rows, a plurality of signal lines extending alongcolumns, and a plurality of pixels disposed in a matrix at points atwhich the scanning lines and the signal lines intersect with each other,wherein, each of the pixels including a load element, a holdingcapacitor, and five N-channel thin film transistors including a samplingtransistor, a drive transistor, a switching transistor, a firstdetection transistor and a second detection transistor, the drivetransistor being connected at the gate thereof to an input node, at thesource thereof to an output node and at the drain thereof to apredetermined power supply voltage, the load element being connectedbetween the output node and a predetermined cathode potential, theholding capacitor being connected between the output node and the inputnode, the sampling transistor being connected to the input node and thesignal line, the first detection transistor being connected between theoutput node and a first ground potential, the second detectiontransistor being connected between the input node and a second groundpotential, the switching transistor being interposed between the inputnode and the gate of the drive transistor, the sampling transistoroperating, when selected by the scanning line, to sample an input signalfrom the signal line and supply the sampled input signal so as to beheld into the holding capacitor, the switching transistor being renderedconducting, when selected by the scanning line, to connect the holdingcapacitor to the gate of the drive transistor, the drive transistordriving the load element with current in response to the signalpotential held by the holding capacitor, and the first and seconddetection transistors operating, when selected by the scanning line, todetect a threshold voltage of the drive transistor prior to the drivingof the load element with the current and supply the detected potentialso as to be held into the holding capacitor in order to cancel aninfluence of the threshold voltage in advance.

According to a yet further embodiment of the present invention, there isprovided a display apparatus including a plurality of scanning linesextending along rows, a plurality of signal lines extending alongcolumns, and a plurality of pixels disposed in a matrix at points atwhich the scanning lines and the signal lines intersect with each other,wherein, each of the pixels including an organic electroluminescenceelement, a holding capacitor, and five N-channel thin film transistorsincluding a sampling transistor, a drive transistor, a switchingtransistor, a first detection transistor and a second detectiontransistor, the drive transistor being connected at the gate thereof toan input node, at the source thereof to an output node and at the drainthereof to a predetermined power supply voltage, the organicelectroluminescence element being connected between the output node anda predetermined cathode potential, the holding capacitor being connectedbetween the output node and the input node, the sampling transistorbeing connected to the input node and the signal line, the firstdetection transistor being connected between the output node and a firstground potential, the second detection transistor being connectedbetween the input node and a second ground potential, the switchingtransistor being interposed between the input node and the gate of thedrive transistor, the sampling transistor operating, when selected bythe scanning line, to sample an input signal from the signal line andsupply the sampled input signal so as to be held into the holdingcapacitor, the switching transistor being rendered conducting, whenselected by the scanning line, to connect the holding capacitor to thegate of the drive transistor, the drive transistor driving the organicelectroluminescence element with current in response to the signalpotential held by the holding capacitor, and the first and seconddetection transistors operating, when selected by the scanning line, todetect a threshold voltage of the drive transistor prior to the drivingof the organic electroluminescence element with the current and supplythe detected potential so as to be held into the holding capacitor inorder to cancel an influence of the threshold voltage in advance.

According to the present invention, the pixel circuit includes anelectro-optical element, a holding capacitor, and five N-channel thinfilm transistors including a sampling transistor, a drive transistor, aswitching transistor, a first detection transistor and a seconddetection transistor. The pixel circuit has a bootstrap function of theholding capacitor, and therefore, even if the I-V characteristic of anelectro-optical element of a current driven type such as a lightemitting element varies with time, the luminance of light emission canbe kept fixed. Further, the threshold voltage of the drive transistor isdetected by the first and second detection transistors, and thevariation of the threshold voltage of the drive transistor iscompensated for by the circuit means. Consequently, the electromagneticelement can be driven stably. Particularly, the pixel circuit iscomposed of one holding capacitor and five transistors and has areasonable configuration which includes a minimized number of circuitelements. As the number of component elements is small, the yield isenhanced and reduction in cost can be anticipated. In addition,according to the present invention, the switching transistor isconnected between the input node and the gate of the drive transistor.Consequently, the drive transistor can be connected to the power supplypotential directly without the intervention of the switching transistor,and therefore, surplus power consumption can be eliminated. Further,since the switching transistor is connected to the gate of the drivetransistor, a high current supplying capacity is not required, andconsequently, miniaturization can be anticipated.

The above and other objects, features and advantages of the presentinvention will become apparent from the following description and theappended claims, taken in conjunction with the accompanying drawings inwhich like parts or elements denoted by like reference symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing an example of a pixel circuit;

FIGS. 2A to 2F are circuit diagrams illustrating operation of the pixelcircuit shown in FIG. 1;

FIG. 3 is a timing chart illustrating operation of the pixel circuitshown in FIG. 1;

FIG. 4 is a circuit diagram showing another example of a pixel circuit;

FIG. 5 is a timing chart illustrating operation of the pixel circuitshown in FIG. 4;

FIG. 6 is a circuit diagram showing a configuration of a pixel circuitto which the present invention is applied;

FIG. 7 is a timing chart illustrating operation of the pixel circuitshown in FIG. 6;

FIG. 8 is a circuit diagram showing a configuration of another pixelcircuit to which the present invention is applied;

FIG. 9 is a timing chart illustrating operation of the pixel circuitshown in FIG. 8;

FIG. 10 is a block diagram showing a configuration of a conventionalorganic EL display apparatus;

FIG. 11 is a circuit diagram showing an example of a conventional pixelcircuit;

FIG. 12 is a graph illustrating aged deterioration of a characteristicof an EL element;

FIG. 13 is a circuit diagram showing another example of a conventionalpixel circuit;

FIG. 14 is a timing chart illustrating operation of the pixel circuitshown in FIG. 13; and

FIGS. 15A and 15B are graphs illustrating a working point of a drivetransistor and an EL element.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, preferred embodiments of the present invention are described indetail with reference to the accompanying drawings. For the convenienceof description, a pixel circuit which has a characteristic variationcompensation function (bootstrap function) of a light emitting elementserving as a load element is described first, and then, another pixelcircuit which additionally has a threshold voltage variationcompensation function of a drive transistor is described. Thereafter,further pixel circuits which have such compensation functions asmentioned above while they are composed of a minimized number of circuitcomponents are described. FIG. 1 shows a configuration of a displayapparatus which includes a pixel circuit having a bootstrap functionwhich is a compensation function for a characteristic variation of alight emitting element which is an electro-optical element. It is to benoted that the circuit configuration shown in FIG. 1 is disclosed inJapanese Patent Application No. '2003-146758 filed on May 23, 2003 inJapan by the assignee of the present patent application.

Referring to FIG. 1, the display apparatus 100 shown includes a pixelarray section 102 in which pixel circuits (PXLC) 101 are arranged in amatrix, a horizontal selector (HSEL) 103, a write scanner (WSCN) 104,and a drive scanner (DSCN) 105. The display apparatus 100 furtherincludes signal lines DTL101 to DTL10 n for being selected by thehorizontal selector 103 such that a signal based on luminanceinformation is supplied thereto, scanning lines WSL101 to WSL10 m forbeing selectively driven by the write scanner 104, and scanning linesDSL101 to DSL10 m for being selectively driving by the drive scanner105. It is to be noted that, for the simplified illustration, aparticular configuration of one pixel circuit is shown in FIG. 1.

The pixel circuit 101 includes n-channel TFTs 111 to 115, a capacitorC111, a light emitting element 117 formed from an organic EL element(OLED: Organic Light Emitting Diode), and nodes ND111 and ND112.Further, in FIG. 1, reference character DTL101 denotes a signal line,WSL101 a scanning line, and DSL101 another scanning line. Of thecomponents, the TFT 111 serves as a driving field effect transistor; thesampling TFT 115 serves as a first switch; the TFT 114 serves as asecond switch; and the capacitor C111 servers as a holding capacitanceelement.

In the pixel circuit 101, the light emitting element (OLED) 117 isinterposed between the source of the TFT 111 and a ground potential GND.More particularly, the anode of the light emitting element 117 isconnected to the source of the TFT 111, and the cathode side of thelight emitting element 117 is connected to the ground potential GND. Thenode ND111 is formed from a connecting point between the anode of thelight emitting element 117 and the source of the TFT 111. The source ofthe TFT 111 is connected to the drain of the TFT 114 and a firstelectrode of the capacitor C111, and the gate of the TFT 111 isconnected to the node ND112. The source of the TFT 114 is connected to afixed potential (in the present embodiment, the ground potential GND),and the gate of the TFT 114 is connected to the scanning line DSL101.The second electrode of the capacitor C111 is connected to the nodeND112. The source and the drain of the sampling TFT 115 are connected tothe signal line DTL101 and the node ND112, respectively. The gate of theTFT 115 is connected to the scanning line WSL101.

In this manner, the pixel circuit 101 according to the presentembodiment is configured such that the capacitor C111 is connectedbetween the gate and the source of the TFT 111 serving as drivetransistor so that the source potential of the TFT 111 is connected tothe fixed potential through the TFT 114 serving as a switchingtransistor.

Now, operation of the display apparatus 100 having the configurationdescribed above is described with reference to FIGS. 2A to 2F and 3A to3F principally in connection to operation of the pixel circuit. It is tobe noted that FIG. 3A illustrates a scanning signal ws [1] applied tothe scanning line WSL101 of the first row of the pixel array; FIG. 3Billustrates another scanning signal ws [2] applied to the scanning lineWSL102 of the second row of the pixel array; FIG. 3C illustrates adriving signal ds[1] applied to the scanning line DSL101 of the firstrow of the pixel array; FIG. 3D illustrates another driving signal ds[2]applied to the scanning line DSL102 of the second row of the pixelarray; FIG. 3E illustrates a gate potential Vg (node ND112) of the TFT111; and FIG. 3F illustrates the source potential Vs (node ND111) of theTFT 111.

First, in an ordinary light emitting state of the EL light emittingelement 117, the scanning signals ws [1], ws [2], to the scanning linesWSL101, WSL102, are selectively set to the low level by the writescanner 104 and the driving signals ds[1], ds[2], to the scanning linesDSL101, DSL102, are selectively set to the low level by the drivescanner 105 as seen in FIGS. 3A to 3D. As a result, in the pixel circuit101, the TFT 115 and the TFT 114 are held in an off state as seen inFIG. 2A.

Then, within a no-light emission period of the EL light emitting element117, the scanning signals ws [1], ws [2], to the scanning lines WSL101,WSL102, are held at the low level by the write scanner 104 and thedriving signals ds[1], ds[2], to the scanning lines DSL101, DSL102, areselectively set to the high level by the drive scanner 105 as seen inFIGS. 3A to 3D. As a result, in the pixel circuit 101, the TFT 114 isturned on while the TFT 115 is held in an off state as seen in FIG. 2B.Thereupon, current flows through the TFT 114, and the source potentialVs of the TFT 111 drops down to the ground potential GND as seen in FIG.3F. Therefore, also the voltage applied to the light emitting element117 drops to 0 V, and the light emitting element 117 is placed into ano-light emission state.

Thereafter, while the driving signals ds[1], ds[2], to the scanninglines DSL101, DSL102, are kept at the high level by the drive scanner105, the scanning signals ws [1], ws [2], to the scanning lines WSL101,WSL102, are selectively set to the high level by the write scanner 104as seen in FIGS. 3A to 3D. As a result, in the pixel circuit 101, whilethe TFT 114 is held in an on state, the TFT 115 is placed into an onstate as seen in FIG. 2C. Consequently, an input signal (Vin)propagating to the signal line DTL101 by the horizontal selector 103 iswritten into the capacitor C111 as a holding capacitor. At this time,since the source potential Vs of the TFT 111 as a drive transistor isequal to the ground potential level (GND level), the potentialdifference between the gate and the source of the TFT 111 is equal tothe signal Vin of the input signal.

Thereafter, within the no-light emission period of the light emittingelement 117, while the driving signals ds[1], ds[2], to the scanninglines DSL101, DSL102, are held at the high level by the drive scanner105, the scanning signals ws [1], ws [2], to the scanning lines WSL101,WSL102, are selectively set to the low level by the write scanner 104 asseen in FIGS. 3A to 3D. As a result, in the pixel circuit 101, the TFT115 is placed into an off state as seen in FIG. 2D, and the writing ofthe input signal into the capacitor C111 as a holding capacitor iscompleted therewith.

Thereafter, the scanning signals ws[1], ws [2], to the scanning linesWSL101, WSL102, are held at the low level by the write scanner 104 andthe driving signals ds[1], ds[2], to the scanning lines DSL101, DSL102,are selectively set to the low level by the drive scanner 105 as seen inFIGS. 3A to 3D. As a result, in the pixel circuit 101, the TFT 114 isplaced into an off state as seen in FIG. 2E. After the TFT 114 is placedinto an off state, the source potential Vs of the TFT 111 as a drivetransistor rises, and current flows also to the light emitting element117.

Although the source potential Vs of the TFT 111 varies, the gate-sourcevoltage is normally held at the voltage Vin as seen in FIGS. 3E and 3F.At this time, since the TFT 111 as a drive transistor operates in asaturation region, the current value Ids flowing through the TFT 111depends upon the voltage Vin which is the gate-source voltage of the TFT111. The current Ids flows also to the light emitting element 117similarly, and consequently, the light emitting element 117 emits light.An equivalent circuit of the light emitting element 117 is shown in FIG.2F, and consequently, the potential at the node ND111 rises up to thegate potential with which the current Ids flows through the EL lightemitting element 117. As the potential rises in this manner, also thepotential at the node ND112 rises similarly through the capacitor C111(holding capacitor). Consequently, the gate-source voltage of the TFT111 is held at the voltage Vin as described hereinabove.

Usually, the I-V characteristic of an EL light emitting elementdeteriorates as the time of light emission therefrom increases.Therefore, even if the drive transistor supplies current of an equalvalue, the potential applied to the EL light emitting element varies andthe potential at the node ND111 drops. However, in the present circuit,since the potential at the node ND111 drops while the gate-sourcevoltage of the drive transistor is kept fixed, current to flow to thedrive transistor (TFT 111) does not change. Consequently, also thecurrent flowing to the EL light emitting element does not change, andeven if the I-V characteristic of the EL light emitting elementdeteriorates, current corresponding to the input voltage Vin continuesto flow.

As described above, in the present form for reference of the pixelcircuit, the source of the TFT 111 as a drive transistor is connected tothe anode of the light emitting element 117 while the drain of the TFT111 is connected to the power supply potential Vcc, and the capacitorC111 is connected between the gate and the source of the TFT 111 suchthat the source potential of the TFT 111 is connected to the fixedpotential through the TFT 114 as a switch transistor. Consequently, thefollowing advantages can be anticipated. In particular, even if the I-Vcharacteristic of the EL light emitting element varies with time, asource follower output free from deterioration in luminance can beobtained. Further, a source follower circuit of n-channel transistorscan be implemented, and an n-channel transistor can be used as a drivingelement for the EL light emitting element while existing anode andcathode electrodes are used. Further, the transistors of the pixelcircuit can be formed only from n-channel transistors, and consequently,the a-Si process can be used in TFT production. As a result, productionof a TFT at a low cost can be anticipated.

FIG. 4 shows a configuration of a pixel circuit wherein a thresholdvoltage cancellation function is additionally provided for the pixelcircuit having a bootstrap function described hereinabove with referenceto FIG. 1. The pixel circuit shown in FIG. 4 is same as the pixelcircuit disclosed in Japanese Patent Application No. 2003-159646 filedon Jun. 4, 2003 in Japan and assigned to the same assignee of thepresent patent application. The pixel circuit of FIG. 4 is basicallyformed from the pixel circuit of FIG. 1 to which a threshold voltagecancellation circuit is added. However, the scanning line WSL101 isconnected in place of the scanning line DSL101 to the gate of the TFT114 included in the bootstrap circuit to simplify the circuitconfiguration. It is basically necessary only to control the TFT 114included in the bootstrap circuit so as to be switched on and off insynchronism with sampling of a video signal, and therefore, suchsimplification as described above is permitted. Naturally, a scanningline DSL101 for exclusive use may be connected to the gate of the TFT114 similarly as in the example of FIG. 1.

Referring to FIG. 4, the threshold voltage cancellation circuit isbasically includes a drive transistor 111, a switching transistor 112,an additional switching transistor 113, and a capacitor C111. Inaddition to the components of the threshold voltage cancellationcircuit, the pixel circuit shown in FIG. 4 includes a coupling capacitorC112 and a switching transistor 116. The source/drain of theadditionally provided switching transistor 113 are connected between thegate and the drain of the TFT 111. Further, the drain of the switchingtransistor 116 is connected to the drain of the TFT 115, and an offsetvoltage Vofs is supplied to the source of the switching transistor 116.The coupling capacitor C112 is interposed between a node ND114 on theTFT 115 side and the node ND112 on the drive transistor 111 side. Ascanning line AZL101 for the cancellation of a threshold voltage (Vth)is connected to the gates of the switching transistors 113 and 116.

FIG. 5 illustrate operation of the pixel circuit shown in FIG. 4. Thepixel circuit performs threshold voltage Vth correction, signal writingand bootstrap operation in order within a period of one field (if). Thethreshold voltage Vth correction and the signal writing are performedwithin a no-light emission period of if, and the boot strap operation isperformed at the top of a light emission period. Further, within thethreshold voltage Vth correction period, the scanning line AZL101 buildsup to the high level while the scanning line DSL101 remains at the highlevel. Consequently, the switching transistors 112 and 113 are turned onsimultaneously, and therefore, current flows and the potential at thenode ND112 connecting to the gate of the TFT 111 rises. Thereafter, thescanning line DSL101 falls to the low level, and consequently, the lightemitting element 117 is placed into a no-light emitting state.Consequently, charge accumulated at the node ND112 is discharged throughthe switching transistor 113, and the potential at the node ND112 dropsgradually. Then, when the potential difference between the node ND112and the node ND111 becomes equal to the threshold voltage Vth, thecurrent through the TFT 111 stops. As can be seen apparently from FIG.5, the potential difference between the node ND112 and the node ND111corresponds to the gate-source voltage Vgs, and from the expression (1),when Vgs=Vth is reached, the current value Ids becomes equal to 0. As aresult, the threshold voltage Vth between the nodes ND112 and ND111 isheld by the capacitor C111.

Then, the scanning line WSL101 exhibits the high level within a periodof 1H, and within the period, the sampling transistor 115 conducts andwriting of a signal is performed. In particular, a video signal Vsigsupplied to the signal line DTL101 is sampled by the sampling transistor115 and written into the capacitor C111 through the coupling capacitorC112. As a result, the held potential Vin of the capacitor C111 becomesequal to the sum of the threshold voltage Vth written formerly and thevideo signal Vsig. However, the input gain of the video signal Vsig isnot 100% but exhibits some loss.

Thereafter, the scanning line DSL101 builds up to the high level andemission of light is started, and the bootstrap operation is performed.Consequently, the signal potential Vin applied to the gate of the drivetransistor 111 rises by ΔV in accordance with the I-D characteristic ofthe EL light emitting element 117. In this manner, the pixel circuit ofFIG. 4 adds the threshold voltage Vth and the voltage ΔV to the netsignal component applied to the gate of the drive transistor 111. Evenif the threshold voltage Vth and the voltage ΔV vary, since theinfluence of the variation can be cancelled, the light emitting element117 can be driven stably.

FIG. 6 shows a pixel circuit to which the present invention is appliedand which is composed of a number of elements reduced from that of thepixel circuit described hereinabove with reference to FIG. 4. Referringto FIG. 6, the present pixel circuit 101 is disposed at each of pointsat which scanning lines and signal lines intersect with each other andcan be applied to a display apparatus of the active matrix type. Whilethe number of signal lines is only one which is the signal line DTL101,the number of scanning lines is four including scanning lines WSL101,DSL101, AZL101 a and AZL101 b disposed in parallel to each other. Thepixel circuit 101 is composed of five N-channel thin film transistorsincluding an electro-optical element 117, a capacitor C111, a samplingtransistor 115, a drive transistor 111, a switching transistor 112, afirst detection transistor 114 and a second detection transistor 113. Inthis manner, the pixel circuit 101 is composed of one holding capacitorand five transistors, and when compared with the pixel circuit shown inFIG. 4, the number of capacitance elements is smaller by one and alsothe number of transistors is smaller by one. Since the number ofcomponent elements is smaller, the yield can be enhanced and the costcan be reduced as much.

The holding capacitor C111 is connected at one terminal thereof to thesource of the drive transistor 111 and at the other terminal thereof tothe gate of the drive transistor 111 similarly. In FIG. 6, the gate ofthe drive transistor 111 is represented by the node ND112, and thesource of the drive transistor 111 is represented by the node ND111similarly. Accordingly, the holding capacitor C111 is connected betweenthe node ND111 and the node ND112. The electro-optical element 117 isformed from, for example, an organic EL element of a diode structure andhas an anode and a cathode. The organic EL element 117 is connected atthe anode thereof to the source (node ND111) of the drive transistor 111and at the cathode thereof to a predetermined cathode potential Vcath.It is to be noted that the organic EL element 117 includes a capacitancecomponent between the anode and the cathode thereof, and the capacitancecomponent is represented by Cp.

The first detection transistor 114 is connected at the source thereof toa first ground potential Vss1 and at the drain thereof to the source(node ND111) of the drive transistor 111. The first detection transistor114 is further connected at the gate thereof to a scanning line AZL101a. The second detection transistor 113 is connected at the sourcethereof to a second ground potential Vss2 and at the drain thereof tothe gate (node ND112) of the drive transistor 111. Further, the seconddetection transistor 113 is connected at the gate thereof to a scanningline AZL101 b.

The sampling transistor 115 is connected at the source thereof to thesignal line DTL101, at the drain thereof to the gate (node ND112) of thedrive transistor 111 and at the gate thereof to the scanning lineWSL101. The switching transistor 112 is connected at the drain thereofto the power supply potential Vcc, at the source thereof to the drain ofthe drive transistor 111, and at the gate thereof to the scanning lineDSL101. The scanning lines AZL101 a, AZL101 b and DSL101 are disposed inparallel to the scanning line WSL101 and are scanned line sequentiallyat suitable timings by the peripheral scanners.

The sampling transistor 115 operates when it is selected by the scanningline WSL101 to sample an input signal Vsig from the signal line DTL101and place the sampled input signal Vsig into the holding capacitor C111through the node ND112. The drive transistor 111 drives theelectro-optical element 117 with current in response to the signalpotential Vin held in the holding capacitor C111. The switchingtransistor 112 is rendered conducting when it is selected by thescanning line DSL101 to supply current from the power supply potentialVcc to the drive transistor 111. The first detection transistor 114 andthe second detection transistor 113 operate when they are selected bythe scanning lines AZL101 a and AZL101 b, respectively, to detect thethreshold voltage Vth of the drive transistor 111 prior to the currentdriving of the electro-optical element 117 and place the detectedpotential into the holding capacitor C111 in order to cancel aninfluence of the threshold voltage Vth.

As a condition for securing normal operation of the pixel circuit 101,the first ground potential Vss1 is set lower than a level calculated bysubtracting the threshold voltage Vth of the drive transistor from thesecond ground potential Vss2. In other words, the first ground potentialVss1 is set so as to satisfy Vss1<Vss2−Vth. Further, a level calculatedby adding a threshold voltage VthEL of the organic EL element 117 to thecathode potential Vcath is set higher than another level calculated bysubtracting the threshold voltage Vth of the drive transistor 111 fromthe first ground potential Vss1. Where this is represented by anexpression, Vcath+VthEL>Vss1−Vth. Preferably, the level of the secondground potential Vss2 is set to a value in the proximity of the lowestlevel of the input signal Vsig supplied from the signal line DTL101.

Operation of the pixel circuit shown in FIG. 6 is described in detailwith reference to a timing chart of FIG. 7. The timing chart of FIG. 7is represented such that one field (1F) starts at timing T1 and ends atanother timing T6. At timing T0 before the field is entered, thescanning lines WSL101, AZL101 a and AZL101 b have the low level whilethe scanning line DSL101 has the high level. Accordingly, the switchingtransistor 112 is in an on state while the sampling transistor 115 andthe detection transistors 113 and 114 in pair are in an off state. Atthis time, the drive transistor 111 supplies driving current in responseto the signal potential appearing at the node ND112 to energize theelectro-optical element 117 to emit light. At this time, the sourcepotential of the drive transistor 111 (potential at the node ND111) isheld at a predetermined working point. The timing chart of FIG. 7illustrates the potential at the node ND112 and the potential at thenode ND111, which represent variation of the gate potential and thesource potential of the drive transistor 111, respectively.

At timing T1, both of the scanning lines AZL101 a and AZL101 b build upfrom the low level to the high level. As a result, both of the firstdetection transistor 114 and the second detection transistor 113 changeover from an off state to an on state. As a result, the potential at thenode ND112 drops to the second ground potential Vss2 quickly, and alsothe potential at the node ND111 drops to the first ground potential Vss1quickly. At this time, since the first ground potential Vss1 and thesecond ground potential Vss2 are set so as to satisfy Vss1<Vss2−Vth asdescribed hereinabove, the drive transistor 111 keeps the on state anddrain current Ids flows. At this time, since the relationship ofVcath+Vth(EL)>Vss1−Vth is satisfied, the organic EL element 117 is in areversely biased state and no current flows therethrough. Accordingly,the organic EL element 117 is placed into a no-light emission state. Thedrain current Ids of the drive transistor 111 flows to the first groundpotential Vss1 side through the first detection transistor 114 which isin an on state.

Then at timing T2, the scanning line AZL101 a changes over from the highlevel to the low level, and consequently, the first detection transistor114 changes over from an on state to an off state. As a result, thecurrent path of the drain current Ids flowing through the drivetransistor 111 is interrupted, and the potential at the node ND111 risesgradually. When the potential difference between the potential at thenode ND111 and the potential at the node ND112 becomes equal to thethreshold voltage Vth, the drive transistor 111 changes over from an onstate to an off state and the drain current Ids stops. The potentialdifference Vth appearing between the node ND111 and the node ND112 isheld by the holding capacitor C111. In this manner, the first and seconddetection transistors 114 and 113 operate when they are selected atsuitable timings by the scanning lines AZL101 a and AZL101 b,respectively, and detect the threshold voltage Vth of the drivetransistor 111 and place the threshold voltage Vth into the holdingcapacitor C111.

Thereafter, at timing T3, the scanning line AZL101 b changes over fromthe high level to the low level, and also the scanning line DSL101changes over from the high level to the low level at a substantiallysame timing. As a result, the second detection transistor 113 and theswitching transistor 112 change over from an on state to an off state.On the timing chart of FIG. 7, the period of time from timing T2 totiming T3 is denoted by Vth correction period, and the detectedthreshold voltage Vth of the drive transistor 111 is held as acorrection potential in the holding capacitor C111.

Thereafter, at timing T4, the scanning line WSL101 builds up from thelow level to the high level. Consequently, the sampling transistor 115is rendered conducting, and the input potential Vin is written into theholding capacitor C111. The input potential Vin is held in such a formthat it is added to the threshold voltage Vth of the driving transistor.As a result, the variation of the threshold voltage Vth of the drivetransistor 111 is always cancelled, and therefore, this signifies thatVth correction is performed. It is to be noted that the input potentialVin written into the holding capacitor C111 is represented by thefollowing expression:Vin=Cp/(Cs+Cp)×(Vsig−Vss 2)where Cs is the capacitance value of the holding capacitor C111, and Cpthe capacitance component of the organic EL element 117 as describedhereinabove. Usually, the capacitance component Cp of the organic ELelement 117 is much higher than the capacitance value Cs of the holdingcapacitor C111. Accordingly, the input potential Vin is substantiallyequal to Vsig−Vss2. In this instance, if the second ground potentialVss2 is set to a value in the proximity of the black level of the inputsignal Vsig, then the input signal Vin becomes substantially equal tothe input signal Vsig.

Thereafter, the scanning line WSL101 changes over from the high levelback to the low level thereby to end the sampling of the input signalVsig. Then at timing T5, the scanning line DSL101 builds up from the lowlevel to the high level and the switching transistor 112 is placed intoan on state. Consequently, driving current is supplied from the powersupply potential Vcc to the drive transistor 111 to start light emittingoperation of the organic EL element 117. Since current flows through theorganic EL element 117, a voltage drop appears and the potential at thenode ND111 rises. In response to the potential rise, also the potentialat the node ND112 rises, and consequently, the gate potential Vgs of thedrive transistor 111 is always kept at Vin+Vth irrespective of thepotential rise at the node ND111. As a result, the organic EL element117 continues to emit light with a luminance corresponding to the inputvoltage Vin. When the scanning lines AZL101 a and AZL101 b build up attiming T6 at the end of the one field, the threshold voltage Vthcorrection period of the next field is entered and also the emission oflight from the electro-optical element 117 stops.

FIG. 8 shows a pixel circuit according to another embodiment of thepresent invention. Referring to FIG. 8, the present pixel circuit 101 isdisposed at each of points at which scanning lines and signal linesintersect with each other and can be applied to a display apparatus ofthe active matrix type. While the number of signal lines is only onewhich is the signal line DTL101, the number of scanning lines is fourincluding scanning lines WSL101, DSL101, AZL101 a and AZL101 b disposedin parallel to each other. The pixel circuit 101 is basically composedof five N-channel thin film transistors including an electro-opticalelement 117, a holding capacitor Cd11, a sampling transistor 115, adrive transistor 111, a switching transistor 112, a first detectiontransistor 114 and a second detection transistor 113. When compared withthe pixel circuit shown in FIG. 1, the number of capacitance elements issmaller by one and also the number of transistors is smaller by one.Since the pixel circuit implemented is composed of one capacitanceelement and five transistors, the yield can be enhanced and the cost canbe reduced when compared with the conventional pixel circuit.

The drive transistor 111 is connected at the gate thereof to the inputnode ND112, at the source thereof connected to the output node ND111,and at the drain thereof to the predetermined power supply potentialVcc. The electro-optical element 117 is formed from an organic ELelement of a diode type and has an anode and a cathode. Theelectro-optical element 117 is connected at the anode thereof to theoutput node ND111 and at the cathode thereof to a predetermined cathodepotential Vcath. The organic EL element 117 includes a capacitancecomponent in parallel to a resistance component, and the capacitancecomponent is represented by Cp. The holding capacitor C111 is connectedbetween the output node ND111 and the input node ND112. The potentialdifference between the output node ND111 and the input node ND112 isjust equal to the gate potential Vgs of the drive transistor 111. Thesampling transistor 115 is connected at the source thereof to the signalline DTL101, at the drain thereof to the input node ND112, and at thegate thereof to the scanning line WSL101.

The first detection transistor 114 is connected at the source thereof tothe first ground potential Vss1, at the drain thereof to the output nodeND111, and at the gate thereof to the scanning line AZL101 a. The secondswitching transistor 113 is connected at the source thereof to thesecond ground potential Vss2, at the drain thereof to the input nodeND112, and at the gate thereof to the scanning line AZL101 b. Theswitching transistor 112 is connected at the source/drain thereofbetween the input node ND112 and the gate of the drive transistor 111.The switching transistor 112 is connected at the gate thereof to thescanning line DSL101. While, in the example for reference shown in FIG.4, the switching transistor is connected between the power supplypotential Vcc and the drive transistor, in the present embodiment, theswitching transistor 112 is connected between the input node and thegate of the drive transistor. According to the present embodiment, sincethe drive transistor 111 can be connected directly to the power supplypotential Vcc, surplus power consumption can be avoided. Further, sincethe switching transistor 112 is connected to the gate of the drivetransistor 111, it need not have a high current supplying capacity andhence can be miniaturized.

The sampling transistor 115 operates when it is selected by the scanningline WSL101 to sample the input signal Vsig from the signal line DTL101and place the sampled input signal Vsig into the holding capacitor C111.The switching transistor 112 is rendered conducting when it is selectedby the scanning line DSL101 to connect the holding capacitor C111 to thegate of the drive transistor 111. The drive transistor 111 drives theelectro-optical element 117 with current in response to the signalpotential Vin held in the holding capacitor C111. The first detectiontransistor 114 and the second switching transistor 113 operate when theyare selected by the different scanning lines AZL101 a and AZL101 b,respectively, to detect the threshold voltage Vth of the drivetransistor 111 prior to the current driving of the electro-opticalelement 117 and place the detected potential into the holding capacitorC111 in order to cancel an influence of the threshold voltage Vth inadvance. Consequently, even if the threshold voltage Vth varies, sincethe variation is always canceled, the drive transistor 111 can supplyfixed drain current Ids to the organic EL element 117 without beinginfluenced by the variation of the threshold voltage Vth.

In order to cause the pixel circuit 101 to operate normally, thepotential relationship must be set correctly. To this end, the firstground potential Vss1 is set lower than a level calculated bysubtracting the threshold voltage Vth of the drive transistor from thesecond ground potential Vss2. This can be represented by an expressionas Vss1<Vss2−Vth. Further, a level calculated by adding the thresholdvoltage VthEL of the organic EL element 117 to the cathode potentialVcath is set higher than another level calculated by subtracting thethreshold voltage Vth of the drive transistor from the first groundpotential Vss1. Where this is represented by an expression,Vcath+VthEL>Vss1−Vth. The expression represents that the organic ELelement 117 is placed into a reversely biased state. Preferably, thelevel of the second ground potential Vss2 is set to a value in theproximity of the lowest level of the input signal Vsig supplied from thesignal line DTL101. Where the capacitance value of the holding capacitorC111 is represented by Cs, the signal potential Vin held by the holdingcapacitor C111 is represented by the following expression:Vin=(Vsig−Vss 2)×(Cp/(Cs+Cp))

The capacitance component Cp of the organic EL element 117 is muchhigher than the value Cs of the holding capacitor, and the signalpotential Vin is substantially equal to Vsig−Vss2. Here, since thesecond ground potential Vss2 is set to a level in the proximity of thelowest level of the input signal Vsig, the signal potential Vin held bythe holding capacitor C111 is substantially equal to the net value ofthe input signal Vsig.

Operation of the pixel circuit shown in FIG. 8 is described in detailwith reference to FIG. 9. The timing chart of FIG. 9 indicates levelvariation of the four scanning lines WSL101, DSL101, AZL101 a and AZL101b within a period of one field (1F). The timing chart further indicatespotential variation at the input node ND112 and the output node ND111 ofthe drive transistor 111 within a period of one field. One field (1F)starts at timing T1 and ends at another timing T6.

At timing T0 before the field is entered, the scanning line DSL101 hasthe high level while the remaining scanning lines WSL101, AZL101 a andAZL101 b have the low level. Accordingly, the switching transistor 112is in an on state while the remaining sampling transistor 115, firstdetection transistor 114 and second detection transistor 113 are in anoff state. In this state, the signal potential Vin held by the holdingcapacitor C111 is applied to the gate of the drive transistor 111through the switching transistor 112 which is in a conducting state.Accordingly, the drive transistor 111 supplies drain current Ids inaccordance with the signal potential Vin to the organic EL element 117.As a result, the organic EL element 117 emits light with a luminancecorresponding to the video signal Vsig.

Then at timing T1, both of the scanning lines AZL101 a and AZL101 bchange over from the low level to the high level at the same time. As aresult, both of the first detection transistor 114 and the seconddetection transistor 113 are turned on simultaneously. As the seconddetection transistor 113 is turned on, the potential at the input nodeND112 drops to the second ground potential Vss2 quickly. Further, as thefirst detection transistor 114 is turned on, the potential at the outputnode ND111 drops down to the first ground potential Vss1 quickly. As aresult, while the gate potential Vgs of the drive transistor 111 isgiven by Vss2−Vss1, since this value is higher than the thresholdvoltage Vth of the drive transistor 111, the drive transistor 111 keepsthe on state and the drain current Ids flows. On the other hand, sincethe potential at the output node ND111 drops to the first groundpotential Vss1, the organic EL element 117 is placed into a reverselybiased state and no current flows therethrough. Accordingly, the organicEL element 117 is placed into a no-light emission state. The draincurrent Ids of the drive transistor 111 flows to the first groundpotential Vss1 through the first detection transistor 114 which is in anon state.

Then at timing T2, the scanning line AZL101 a changes over from the highlevel to the low level, and consequently, the first detection transistor114 is placed into an off state. As a result, the current path to thedrive transistor 111 is interrupted, and the potential at the outputnode ND111 rises gradually. When the potential difference between theoutput node ND111 and the input node ND112 becomes equal to thethreshold voltage Vth of the drive transistor 111, the current becomesequal to zero and the threshold voltage Vth is held by the holdingcapacitor C111 connected between the nodes ND112 and ND111. Thethreshold voltage Vth of the drive transistor 111 is detected by thepair of detection transistors 113 and 114 and held by the holdingcapacitor C111 in this manner. The period of time from timing T2 totiming T3 within which the operation described above is performed isrepresented by a Vth correction period. It is to be noted that thetiming T3 represents a timing at which the scanning line DSL101 and thescanning line AZL101 b change over from the high level to the low levelafter the current reduces to zero. As a result, the switching transistor112 is placed once into an off state and also the second detectiontransistor 113 is placed into an off state. Consequently, the input nodeND112 is disconnected from the gate of the drive transistor 111 and alsofrom the second ground potential Vss2 and therefore can thereafterperform sampling operation.

At timing T4, the scanning line WSL101 builds up to the high level andthe sampling transistor 115 is turned on. Consequently, the input signalVsig supplied from the signal line DTL101 is sampled, and the inputpotential Vin which is substantially equal to the net value of the inputsignal Vsig is written into the holding capacitor C111. The inputpotential Vin is held in such a form that it is added to the thresholdvoltage Vth held formerly.

At timing T5 after the sampling of the video signal Vsig comes to an endin this manner, the scanning line DSL101 builds up to the high levelagain and the switching transistor 112 is placed into an on state sothat emission of light from the organic EL element 117 is started. Inparticular, the input potential Vin held in the holding capacitor C111is applied to the gate of the drive transistor 111 through the switchingtransistor 112. The drive transistor 111 supplies the drain current Idsin accordance with the input potential Vin to the organic EL element 117to start emission of light from the organic EL element 117. After thecurrent begins to flow through the organic EL element 117, a voltagedrop occurs, and the level at the output node ND111 begins to rise.Simultaneously, since also the level at the input node ND112 begins torise, the potential Vin+Vth held in the holding capacitor C111 remainsfixed. By such a bootstrap operation as described above, even if thelevel at the output node ND111 varies by variation of the working pointof the organic EL element 117, the drive transistor 111 can supplynormally fixed drain current Ids. At timing T6, the scanning linesAZL101 a and AZL101 b build up finally, and threshold voltage Vthdetection operation for a next field is started.

While preferred embodiments of the present invention have been describedusing specific terms, such description is for illustrative purpose only,and it is to be understood that changes and variations may be madewithout departing from the spirit or scope of the following claims.

1. A pixel circuit disposed at a point at which a scanning line and asignal line intersect with each other, comprising: an electro-opticalelement; a holding capacitor; and five N-channel thin film transistorsincluding a sampling transistor, a drive transistor, a switchingtransistor, a first detection transistor and a second detectiontransistor; wherein, said holding capacitor being connected between thesource and the gate of said drive transistor; said electro-opticalelement being connected between the source of said drive transistor anda predetermined cathode potential; said first detection transistor beingconnected between the source of said drive transistor and a first groundpotential; said second detection transistor being connected between thegate of said drive transistor and a second ground potential; saidsampling transistor being connected between said gate of said drivetransistor and said signal line; said switching transistor beingconnected between the drain of said drive transistor and a predeterminedpower supply potential; said sampling transistor operating, whenselected by said scanning line, to sample an input signal from saidsignal line and supply the sampled input signal so as to be held intosaid holding capacitor; said drive transistor driving saidelectro-optical element with current in response to the signal potentialheld by said holding capacitor; said switching transistor being renderedconducting, when selected by said scanning line, to supply current fromsaid power supply potential to said drive transistor; and said first andsecond detection transistors operating, when selected by said scanningline, to detect a threshold voltage of said drive transistor prior tothe driving of said electro-optical element with the current and supplythe detected potential so as to be held into said holding capacitor inorder to cancel an influence of the threshold voltage in advance.
 2. Thepixel circuit according to claim 1, wherein said first ground potentialis set lower than a level calculated by subtracting the thresholdvoltage of said drive transistor from said second ground potential, anda level calculated by adding the threshold voltage of saidelectro-optical element to said cathode potential is set higher than alevel calculated by subtracting the threshold voltage of said drivetransistor from said first ground potential.
 3. The pixel circuitaccording to claim 2, wherein the level of said second ground potentialis set to a value in the proximity of the lowest level of the inputsignal supplied from said signal line.
 4. An active matrix apparatus,comprising: a plurality of scanning lines extending along rows; aplurality of signal lines extending along columns; and a plurality ofpixels disposed in a matrix at points at which said scanning lines andsaid signal lines intersect with each other; wherein, each of saidpixels including a load element, a holding capacitor, and five N-channelthin film transistors including a sampling transistor, a drivetransistor, a switching transistor, a first detection transistor and asecond detection transistor; said holding capacitor being connectedbetween the source and the gate of said drive transistor; said loadelement being connected between the source of said drive transistor anda predetermined cathode potential; said first detection transistor beingconnected between the source of said drive transistor and a first groundpotential; said second detection transistor being connected between thegate of said drive transistor and a second ground potential; saidsampling transistor being connected between said gate of said drivetransistor and said signal line; said switching transistor beingconnected between the drain of said drive transistor and a predeterminedpower supply potential; said sampling transistor operating, whenselected by said scanning line, to sample an input signal from saidsignal line and supply the sampled input signal so as to be held intosaid holding capacitor; said drive transistor driving said load elementwith current in response to the signal potential held by said holdingcapacitor; said switching transistor being rendered conducting, whenselected by said scanning line, to supply current from said power supplypotential to said drive transistor; and said first and second detectiontransistors operating, when selected by said scanning line, to detect athreshold voltage of said drive transistor prior to the driving of saidload element with the current and supply the detected potential so as tobe held into said holding capacitor in order to cancel an influence ofthe threshold voltage in advance.
 5. A display apparatus, comprising: aplurality of scanning lines extending along rows; a plurality of signallines extending along columns; and a plurality of pixels disposed in amatrix at points at which said scanning lines and said signal linesintersect with each other; wherein, each of said pixels including anorganic electroluminescence element, a holding capacitor, and fiveN-channel thin film transistors including a sampling transistor, a drivetransistor, a switching transistor, a first detection transistor and asecond detection transistor; said holding capacitor being connectedbetween the source and the gate of said drive transistor; said organicelectroluminescence element being connected between the source of saiddrive transistor and a predetermined cathode potential; said firstdetection transistor being connected between the source of said drivetransistor and a first ground potential; said second detectiontransistor being connected between the gate of said drive transistor anda second ground potential; said sampling transistor being connectedbetween said gate of said drive transistor and said signal line; saidswitching transistor being connected between the drain of said drivetransistor and a predetermined power supply potential; said samplingtransistor operating, when selected by said scanning line, to sample aninput signal from said signal line and supply the sampled input signalso as to be held into said holding capacitor; said drive transistordriving said organic electroluminescence element with current inresponse to the signal potential held by said holding capacitor; saidswitching transistor being rendered conducting, when selected by saidscanning line, to supply current from said power supply potential tosaid drive transistor; and said first and second detection transistorsoperating, when selected by said scanning line, to detect a thresholdvoltage of said drive transistor prior to the driving of said organicelectroluminescence element with the current and supply the detectedpotential so as to be held into said holding capacitor in order tocancel an influence of the threshold voltage in advance.
 6. A pixelcircuit disposed at a point at which a scanning line and a signal lineintersect with each other, comprising: an electro-optical element; aholding capacitor; and five N-channel thin film transistors including asampling transistor, a drive transistor, a switching transistor, a firstdetection transistor and a second detection transistor; wherein, saiddrive transistor being connected at the gate thereof to an input node,at the source thereof to an output node and at the drain thereof to apredetermined power supply voltage; said electro-optical element beingconnected between said output node and a predetermined cathodepotential; said holding capacitor being connected between said outputnode and said input node; said sampling transistor being connected tosaid input node and said signal line; said first detection transistorbeing connected between said output node and a first ground potential;said second detection transistor being connected between said input nodeand a second ground potential; said switching transistor beinginterposed between said input node and the gate of said drivetransistor; said sampling transistor operating, when selected by saidscanning line, to sample an input signal from said signal line andsupply the sampled input signal so as to be held into said holdingcapacitor; said switching transistor being rendered conducting, whenselected by said scanning line, to connect said holding capacitor to thegate of said drive transistor; said drive transistor driving saidelectro-optical element with current in response to the signal potentialheld by said holding capacitor; and said first and second detectiontransistors operating, when selected by said scanning line, to detect athreshold voltage of said drive transistor prior to the driving of saidelectro-optical element with the current and supply the detectedpotential so as to be held into said holding capacitor in order tocancel an influence of the threshold voltage in advance.
 7. The pixelcircuit according to claim 6, wherein said first ground potential is setlower than a level calculated by subtracting the threshold voltage ofsaid drive transistor from said second ground potential, and a levelcalculated by adding the threshold voltage of said electro-opticalelement to said cathode potential is set higher than a level calculatedby subtracting the threshold voltage of said drive transistor from saidfirst ground potential.
 8. The pixel circuit according to claim 7,wherein the level of said second ground potential is set to a value inthe proximity of the lowest level of the input signal supplied from saidsignal line.
 9. An active matrix apparatus, comprising: a plurality ofscanning lines extending along rows; a plurality of signal linesextending along columns; and a plurality of pixels disposed in a matrixat points at which said scanning lines and said signal lines intersectwith each other; wherein, each of said pixels including a load element,a holding capacitor, and five N-channel thin film transistors includinga sampling transistor, a drive transistor, a switching transistor, afirst detection transistor and a second detection transistor; said drivetransistor being connected at the gate thereof to an input node, at thesource thereof to an output node and at the drain thereof to apredetermined power supply voltage; said load element being connectedbetween said output node and a predetermined cathode potential; saidholding capacitor being connected between said output node and saidinput node; said sampling transistor being connected to said input nodeand said signal line; said first detection transistor being connectedbetween said output node and a first ground potential; said seconddetection transistor being connected between said input node and asecond ground potential; said switching transistor being interposedbetween said input node and the gate of said drive transistor; saidsampling transistor operating, when selected by said scanning line, tosample an input signal from said signal line and supply the sampledinput signal so as to be held into said holding capacitor; saidswitching transistor being rendered conducting, when selected by saidscanning line, to connect said holding capacitor to the gate of saiddrive transistor; said drive transistor driving said load element withcurrent in response to the signal potential held by said holdingcapacitor; and said first and second detection transistors operating,when selected by said scanning line, to detect a threshold voltage ofsaid drive transistor prior to the driving of said load element with thecurrent and supply the detected potential so as to be held into saidholding capacitor in order to cancel an influence of the thresholdvoltage in advance.
 10. A display apparatus, comprising: a plurality ofscanning lines extending along rows; a plurality of signal linesextending along columns; and a plurality of pixels disposed in a matrixat points at which said scanning lines and said signal lines intersectwith each other; wherein, each of said pixels including an organicelectroluminescence element, a holding capacitor, and five N-channelthin film transistors including a sampling transistor, a drivetransistor, a switching transistor, a first detection transistor and asecond detection transistor; said drive transistor being connected atthe gate thereof to an input node, at the source thereof to an outputnode and at the drain thereof to a predetermined power supply voltage;said organic electroluminescence element being connected between saidoutput node and a predetermined cathode potential; said holdingcapacitor being connected between said output node and said input node;said sampling transistor being connected to said input node and saidsignal line; said first detection transistor being connected betweensaid output node and a first ground potential; said second detectiontransistor being connected between said input node and a second groundpotential; said switching transistor being interposed between said inputnode and the gate of said drive transistor; said sampling transistoroperating, when selected by said scanning line, to sample an inputsignal from said signal line and supply the sampled input signal so asto be held into said holding capacitor; said switching transistor beingrendered conducting, when selected by said scanning line, to connectsaid holding capacitor to the gate of said drive transistor; said drivetransistor driving said organic electroluminescence element with currentin response to the signal potential held by said holding capacitor; andsaid first and second detection transistors operating, when selected bysaid scanning line, to detect a threshold voltage of said drivetransistor prior to the driving of said organic electroluminescenceelement with the current and supply the detected potential so as to beheld into said holding capacitor in order to cancel an influence of thethreshold voltage in advance.